Boukyaku no Senritsu

Synopsis

"The 20th Century saw a massive conflict between humans and Monsters, with the Monsters winning the hard-fought struggle. As a new century dawns, memories of those times have utterly faded from the minds of men." Monsters rule from the shadows, while humans continue life with a semblance of normalcy, all the while in secret fear. Some try to win favor with the Monsters, offering sacrifices of children. Most turn a blind eye and pretend they don't exist. But there are a few, the Melos Warriors, who continue to fight the Monsters and seek the Melody of Oblivion, a mysterious girl said to hold the key to salvation. Young Serina Bocca awakens as a Melos Warrior and begins his journey in search of the Melody of Oblivion.

Synopsis & details from MangaUpdates